1. Field of the Disclosure
The present invention relates generally to power supplies, and more specifically, the invention relates to control circuits to regulate an output of a power supply by measuring a quantity of charge received from the power supply input.
2. Background
In a typical switched-mode power supply application, the ac-dc power supply receives an input that is between 100 and 240 volts rms from an ordinary ac electrical outlet. Switches in the power supply are switched on and off by a control circuit to provide a regulated output that may be suitable for operating an electronic device, or for charging a battery that provides power to an electronic device. The regulated output is typically a dc voltage less than 10 volts dc. Furthermore, the current from the output is usually regulated when the power supply is charging a battery.
Safety agencies generally require the power supply to provide galvanic isolation between the input and the output of the power supply. Galvanic isolation prevents dc current from flowing between the input and the output of the power supply. In other words, a high dc voltage applied between an input terminal and an output terminal of the power supply will produce no dc current between the input terminal and the output terminal of the power supply. The requirement for galvanic isolation is a complication that contributes to the cost of the power supply.
A power supply with galvanic isolation must maintain an isolation barrier that electrically separates the input from the output. Energy must be transferred across the isolation barrier to provide power to the output, and information in the form of feedback signals in many cases is transferred across the isolation barrier to regulate the output. Galvanic isolation is typically achieved with electromagnetic and electro-optical devices. Electromagnetic devices such as transformers and coupled inductors are generally used to transfer energy between input and output to provide output power, whereas electro-optical devices are generally used to transfer signals between output and input to control the transfer of energy between input and output.
Efforts to reduce the cost of the power supply have focused on the elimination of electro-optical devices and their associated circuits. Alternative solutions generally use a single energy transfer element such as, for example, a transformer or, for example, a coupled inductor to provide energy to the output and also to obtain the information necessary to control the output. The lowest cost configuration typically places the control circuit and a high voltage switch on the input side of the isolation barrier. The controller obtains information about the output indirectly from observation of a voltage at a winding of the energy transfer element. The winding that provides the information is also on the input side of the isolation barrier. To reduce cost and complexity further, the controller can also use the same winding of the energy transfer element to obtain information about the input to the power supply to control an output of the power supply.
The input side of the isolation barrier is sometimes referred to as the primary side, and the output side of the isolation barrier is sometimes referred to as the secondary side. Windings of the energy transfer element that are not galvanically isolated from the primary side are also primary side windings, sometimes called primary referenced windings. A winding on the primary side that is coupled to an input voltage and receives energy from the input voltage is sometimes referred to simply as the primary winding. Other primary referenced windings that deliver energy to circuits on the primary side may have names that describe their principal function, such as for example a bias winding, or for example a sense winding. Windings that are galvanically isolated from the primary side windings are secondary side windings, sometimes called output windings.
While it is quite straightforward to use a winding on the input side of the isolation barrier to obtain information indirectly about a galvanically isolated output voltage, it is a different challenge to obtain information indirectly about a galvanically isolated output current. In many power supply topologies, the measurement of a current in an input winding alone is not sufficient to determine an output current. Conventional solutions for measuring an output current usually include a current to voltage conversion that wastes power and uses costly components to transmit a signal across the isolation barrier.